What is the most efficient way and the industry standard to make and add textures to a UVmap?
The UV Map is not an image itself, it is the mathematical projection of the vertex in an X and Y plane. The image texture is just that, an image texture - but it is aligned to a specific UV mapping so that it can align to the vertices of your mesh. Paint in the 3d view, paint in an external editor, or as now possible paint in Substance Painter and get all your texture images there. Do it how you want to, the result is what is important.